Subaqueous silicification of the contents of small ponds in an Early Devonian hot-spring complex, Rhynie, Scotland

Abstract

The Early Devonian Rhynie and Windyfield cherts of northeast Scotland originated as siliceous sinters deposited by hot springs. Silicification affected both subaerial and subaqueous environments, preserving a diverse terrestrial and freshwater biota. Cherts originally deposited in small shallow pools can be recognised on the basis of both texture and fossil content. Textures comprise clotted and microcoprolitic textures, bacterial coatings on plant axes that can be stromatolitic, and a variety of bacterial and fungal meshworks. The crustacean Lepidocaris, the euthycarcinoid Heterocrania, the charophyte alga Palaeonitella, and chytrid fungi are typical elements of the aquatic biota. Observations of modern hot springs in Yellowstone National Park, Wyoming, U.S.A., demonstrate that shallow ponds and streams on low-angle outwash areas and dormant vent orifices provide suitable environmental analogues. Textures comparable to those described from Rhynie are recorded from Yellowstone sinters, but examples of the rapid and complete silicification of delicate organic structures as seen in a few of the Rhynie chert beds have not been noted. Petrographic textures comparable to those seen in the cherts of freshwater origin from Rhynie occur in modern stream sinters at Yellowstone, where they form from waters at 20–28 °C and with a pH of 8.7. This similarity occurs despite differences in environment at Yellowstone, such as the oxidizing surface environment, water table fluctuations, complex modern vegetation, fixing of silica by diatoms, and climatic extremes. Thus there are close similarities between textures seen in the Rhynie cherts and Yellowstone sinters deposited in freshwater pools and streams by hot springs.

Wood Petrifaction: A New View of Permineralization and Replacement

“…At both localities, fossilization involved silicification of ancient peat bogs, but geochemical conditions at the two sites were very different. The Rhynie Chert involved silica precipitation in a hot spring environment [15][16][17]. The Princeton Chert has been interpreted to result from cyclic fluvial processes that ensued from episodes of tectonic tilting of a floodplain [18].…”

Preservation potential of microorganism morphologies in tufas, sinters, and travertines through geologic time

“…3a, 4a-c, Table 2). These features have been well illustrated and described in numerous recent publications [see figures from Jones and Renaut (1996), Jones et al (2000), Campbell et al (2002), Guidry and Chafetz (2003a, b), Jones and Renaut (2003), Trewin et al (2003), Trewin and Rice (2004), Jones et al (2004Jones et al ( , 2005, and Konhauser (2007) for excellent examples]. Konhauser (2007) describes three basic bacterial shapes as seen under the SEM: spheres are known as microbial coccoids; helical shapes are known as microbial spirilla; rods are known as microbial bacilli.…”

“…Thick filaments of up to 100 μm in diameter were observed in the Devonian sinter samples, but, as noted previously, they likely represent fossilized fungal hyphae (Trewin et al 2003). They would therefore have been thicker than bacterial or cyanobacterial filaments originally, so their preserved diameter may be approximately the same as their original size.…”

“…Thirty rock stubs (small chips) were cut from the most promising (based on thin section recognition of clotted and/or laminated textures, potentially indicative of microbial activity) of the original 50 TST samples, sputter-coated with gold, and then examined on a Zeiss DM 962 (Carl Zeiss SMT, Peabody, MA) SEM with operating conditions between 15-30 kV. Rice et al 2002;Trewin et al 2003 Further details on the Mesozoic tufa, sinter, and travertine (TST) samples are in de Wet et al (2002) Stubs from Holocene tufas and travertines were from broken or cut surfaces, and microbial populations were not living at the time of sample preparation. SEM examination of pre-Holocene TST samples was performed on sawed, unetched rock surfaces.…”

“…All of the threadlike forms observed in this study are interpreted as having formed as permineralized cyanobacterial sheaths and/or extracellular polymeric substances (EPS) from cyanobacterial or bacterial filaments (Whitton and Potts 2000;Pentecost 2003), except the considerably thicker strands (up to 100 μm in diameter) in the Devonian sinter. These features are interpreted as fungal hyphae by Trewin et al (2003). The majority of aquatic microorganisms grow in microcolonies attached to submerged surfaces in the form of biofilms (Konhauser 2007).…”